Electrochemical cell comprising lamination of electrode and paper separator members

ABSTRACT

In double-layer capacitor and rechargeable battery electrochemical cell systems comprising opposed electrode members of polymeric matrix composition having an interposed electrically insulative, ion-conductive separator member incorporating electrolyte solution, thermal lamination of the electrode members to an interposed paper separator member to form a unitary cell structure is enabled by initially providing in the region of the separator/electrode interface, either incorporated into the electrode composition or situated in the separator member, a sufficient amount of a supplemental plasticizer compatible with the electrode matrix polymer to render at least the surface portion of the electrode composition capable of adhesive flow under the selected conditions of laminating heat and pressure. After lamination, a sufficient amount of the supplemental plasticizer is removed, by evaporation or selective extraction, to ensure against delamination of the cell structure in the event of exposure to vagrant heating.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to electrochemical cells, such asdouble-layer capacitors and rechargeable batteries, comprisingopposed-polarity electrode members with an interposed electricallyinsulating, ion-conductive separator member. Particularly, the inventionrelates to such cells in which the electrode members typically comprisepolymeric matrix compositions of electrochemically active materials,which electrode members are laminated, preferably by means of appliedheat and pressure, to the interposed separator member to form a unitarycell body. More particularly, the invention relates to such cells inwhich the interposed separator member comprises a sheet of paper, suchas typically used in the fabrication of canister-type electrochemicaldouble-layer (EDL) capacitor cells, composed of matted fibers ofcellulose, regenerated cellulose, or the like, which are economicallydesirable and have the ability to absorb and retain substantial amountsof commonly employed liquid electrolyte compositions and thereby toprovide high levels of ionic conductivity in the incorporating cell. Inaddition, the invention provides a means for fabricating such laminatedcell structures while maintaining the capability of the paper separatormember to retain an optimum amount of electrolyte and yieldexceptionally high ionic conductivity.

[0002] A number of laminated, polymeric electrode electrochemical cellproducts have previously been described. For example, rechargeablebatteries of this type are discussed in U.S. Pat. No. 5,456,000, whilesupercapacitor systems are detailed in U.S. Pat. No. 6,187,061. However,although these prior systems refer to substantially the same respectiveelectrode compositions as may be utilized in the present invention,these earlier cells were limited to the use of polymeric separatormembers, principally in order to enable thermally activated cohesivelamination between the similar polymeric compositions of the electrodeand separator members. While such polymeric separator members werecapable, with appropriate treatment, of retaining sufficient amounts ofelectrolyte composition to ensure effective operation, the cost of thepolymeric separator materials and their fabrication was considerablyhigher, and, in some applications, e.g., in high-rate EDLsupercapacitors, the level of electrolyte retention and resulting ionicconductivity was significantly less than normally available withexisting low-cost paper separator materials typically used innon-laminated, physically compressed canister cell devices.

[0003] Numerous sheet paper products are currently available for use asseparator members in electrochemical systems. Such products includesingle-sheet cellulose fiber materials, as well as multi-layer productscomprising cellulosic and other composition sheets of various densitywhich ostensibly yield the desirable properties, i.e., electrolyteabsorption, electronic insulation, and physical strength, of therespective component sheet materials. Separator products of this typeare mentioned, for example, in U.S. Pat. No. 6,104,600.

[0004] While such separator papers perform suitably well in the intendedphysical compression type of electrochemical cells, the resistance ofthe surfaces of these papers to interfacial adhesion with desirable cellelectrode compositions, particularly in preferred thermal laminatingprocedures, prevents use of these papers in the fabrication of unitarylaminated cell structures. Attempts to counter this weakness by meansincluding the application of supplementary adhesive compositions at suchsheet interfaces has resulted in the greater disadvantage of occludingthe otherwise advantageous natural porosity of the papers and thusgreatly diminishing electrolyte transport and reducing necessary ionicconductivity of an incorporating cell.

[0005] Similar attempts to utilize coated or in situ adhesivecompositions as a means of laminating single-sheet paper separatormembers with cell electrode members to form unitary cell structures haveresulted in like unacceptably high levels of cell impedance. The presentinvention, on the other hand, provides a means for avoiding priordeleterious obstruction of paper separator pores due to applied adhesivecompositions while enabling the formation of strong, permanent bondingbetween electrochemical cell electrode members and interposed paperseparator members.

SUMMARY OF THE INVENTION

[0006] Electrochemical cell members employed in the present inventionare typical of previous laminated cell components, namely, a pair oflaminar polymeric matrix electrode members of electrochemically activecomponents, commonly cast from fluid coating compositions, and aninterposed ion-transmissive, electron-insulative separator member. Asare normally associated with the electrode members, electricallyconductive current collector members, often of reticulated metallicmaterial, are laminated to or embedded in the electrode members at aconvenient stage in the fabrication procedure.

[0007] In a preferred embodiment of the present invention, opposedelectrode members of an electrochemical cell are prepared withrespective active material components, e.g., activated carbon insupercapacitor fabrications or ion intercalation compounds inrechargeable battery structures, dispersed in a polymeric matrixcomposition, e.g., a poly(vinylidene fluoride) homopolymer or itscopolymer with hexafluoropropylene, chlorotrifluoroethylene, ortetrafluoroethylene incorporated with a compatible primary plasticizer.

[0008] The proportion of such plasticizer is selected to be sufficientto impart to the electrode matrix polymeric component the capability ofthermoadhesive flow at preferred applied temperatures, usually in therange of about 100° C.-140° C. In such a heat-activated state, theelectrode polymer composition is capable of forming under pressure,e.g., with opposed platens or typical lamination rollers at about 20-40N/cm, a physical bond with the surface of a separator member paper ofcellulose, regenerated cellulose, or composite cellulosic type widelyavailable for use in canister style capacitor devices. While theelectrode matrix composition, which initially comprises about 2 to 4parts of primary plasticizer, e.g., propylene carbonate, per part ofcopolymer, exhibits sufficient adhesive flow under such laminationconditions to effect a firm bond to the surface of a separator memberpaper, there is insufficient penetration into the pores of the paper, asis often experienced with applied fluid adhesive compositions, tosignificantly occlude the pores and reduce the desired absorption oflater-introduced electrolyte solution into the separator member.

[0009] In an alternative fabrication procedure, a lesser portion of theelectrode composition plasticizer sufficient to initiate adhesive flowof the electrode matrix polymer at least in the interfacial separatorcontact region under lamination conditions is deposited in the separatorpaper prior to cell member assembly. In this manner, there is no polymerresidue to occlude the separator paper pores after removal ofplasticizer, as described below. For example, a desired amount of theplasticizer, either neat or dissolved in a solvent vehicle which will beremoved prior to lamination, may be applied to the separator paper bycoating, dipping, spraying, or other common means.

[0010] Subsequent to the lamination of electrode members with theinterposed paper separator member to form the unitary cell structure,the laminate bond is rendered resistant to weakening in the event ofsubsequent exposure to vagrant heating by means of removal of the matrixplasticizer component by extraction with a selective solvent, e.g.,diethyl ether, methanol, ethyl acetate, or the like to which the matrixpolymer component is substantially inert, or, with more volatileplasticizer compounds, by evaporation. The unitary cell structure isthen activated by imbibition of typical electrolyte solution prior tohermetic sealing in a packaging member which, due to the unitarylaminated structure of the operating cell, need not take the form ofprior rigid canisters, but may preferably be a variously shaped envelopeof flexible packaging material, typically comprising a foil and polymerfilm laminate.

DESCRIPTION OF THE INVENTION

[0011] The following exemplary compositions and fabrication procedureswill typify the present invention, and, unless otherwise noted,components of the representative compositions are proportioned on aweight basis.

EXAMPLE I

[0012] A supercapacitor embodiment of the present invention was preparedwith 0.125 mm thick electrode member layer material cast from ahomogeneous coating composition of 37.5 parts activated carbon(ASupra-Norit Co.), 2.5 parts conductive carbon black (SP-Erachem Co.),10.0 parts poly(vinylidene fluoride-co-hexafluoropropylene) (PVdF-HFP)(Kynar PowerFLEX LBG -TotalFinaElf), and 50.0 parts propylene carbonate(PC) plasticizer in about 100 parts acetone. Upon air-drying to removethe acetone coating vehicle, a flexible sheet of polymeric matrixactivated carbon electrode material was obtained from which electrodemembers of desired size were readily cut.

[0013] A current collector member of aluminum expanded foil grid(Microgrid-Delker Corp.) was laminated to one surface of each of twoelectrode members in a heated compressive roller device at about 140° C.and 30 N/cm, effectively embedding the grid into the electrode surface.The remaining surfaces of the electrode members were arranged in contactwith the respective surfaces of a sheet of 30 μm, 9 g/m² regeneratedcellulose capacitor separator paper (TF4030-NKK, Japan), and theassemblage was passed through the roller device at about 130° C. and 20N/cm. Upon cooling, the laminated assembly exhibited a strong adhesionbetween the cell members which resulted in paper fiber tear duringattempted separation of the members.

[0014] The laminated unitary supercapacitor cell structure was rinsedonce in diethyl ether to remove by extraction the PC plasticizer and wasdried at about 80° C. under vacuum. The extracted cell laminate was thenhermetically sealed under inert atmosphere in an envelope of polymericpackaging film with a typical supercapacitor electrolyte solution of 1.5M tetraethylammonium tetrafluoroborate in acetonitrile. The resultingsupercapacitor exhibited an impedance of 1.7 ohm.cm² and a capacitanceof 0.48 F.cm⁻², and it performed stably over numerous cycles.

EXAMPLE II

[0015] As a comparative example to display the efficacy of the presentinvention, a supercapacitor electrode composition was prepared as inExample I with the exception that the amount of PC plasticizer wasreduced to a prior typical amount of about 20 parts. The resulting sheetmaterial was sufficiently flexible to form electrode members and tolaminate effectively with embedded aluminum grid current collectormembers or to prior separator members of polymeric composition. However,attempted lamination of the electrode subassemblies to the selectedTF4030 separator paper member under the conditions of Example I providedonly marginally acceptable physical adherence.

[0016] After solvent extraction and activation with the electrolytesolution, the resulting supercapacitor cell initially exhibited testresults comparable to the earlier cell, but these results soon becameerratic due to rapid loss of interfacial adhesion between electrode andseparator members.

EXAMPLE III

[0017] A further comparative example was prepared utilizing theelectrode sheet material of Example II. In order to enhance the physicaladhesion between the electrode members and the paper separator member,the latter was pretreated by application of a polymer solution which wasanticipated to provide a compatible film surface conducive to adhesionwith the polymeric matrix of the electrode member composition. To thisend, the TF4030 separator paper was coated with a 5% acetone solution ofthe same PVdF-HFP copolymer as that used for the electrode matrix.

[0018] The thermal lamination procedures employed previously provide thedesired improvement in physical adhesion between the electrode andseparator members; however, under performance testing the activated cellexhibited an elevated impedance of 1.9 ohm.cm² and a greatly reducedcapacitance of 0.24 F.cm⁻². Subsequent testing of examples of theseparator paper stock pretreated with acetone containing 0% (control),2%, and 5% PVdF-HFP revealed a significant decrease of ionicconductivity of the respective electrolyte activated papers to levelsfrom 5.77 mS/cm (control) to 3.79 mS/cm and 2.22 mS/cm. Thus, althoughthe copolymer taken up by the separator stock paper enhanced theadhesion to the like polymer matrix of the electrode members, theresulting occlusion of the paper pores sufficiently reduced the movementof the critical activating electrolyte solution to thus render ionicconductivity unacceptable for useful supercapacitor fabrication.

EXAMPLE IV

[0019] By way of yet further comparison with prior art practices, a cellwas prepared with the electrode members of Example III and a separatormember of the TF4030 paper which had been pretreated, according to thepresent invention, with a solution of about 20% PC plasticizer in methylalcohol. After removal of the alcohol vehicle by evaporation in air, thePC-containing separator paper member was assembled with the electrodemembers in the previous manner and laminated under the conditions ofExample I. The resulting unitary cell structure, after extraction,activation, and packaging as in that example, performed under testingsubstantially as well as its exemplary cell over an extended test periodand exhibited no tendency for disruptive delamination as was observedwith the plasticizer-deficient fabrication of Example II. As was evidentin this example, the supplemental plasticizer provided at the electrodeinterfaces by the treated separator member was sufficient, with theincorporated electrode composition plasticizer component, to enable theelectrode matrix polymer to achieve thermoplastic adhesion to theseparator paper at the selected lamination conditions. Also, beingreadily removed from the separator member paper, the temporarysupplemental plasticizer poses no threat to occlude the paper pores orotherwise interfere with the advantageous absorption of cell electrolytesolution.

EXAMPLE V

[0020] A rechargeable battery cell embodiment of the present inventionwas prepared with electrode member layers comprising polymeric matrixcompositions of typical active intercalation components, e.g., LiCoO₂and graphite. In accordance with common practice, the thicknesses of theelectrode material layers were adjusted during preparation to obtain inthe final electrode members an active positive to negative materialratio of about 2.1.

[0021] In the present example, positive electrode layer material wascast from a homogeneous coating composition of 79.0 parts commercialgrade LiCoO₂ powder (Grade C022-Seimi Chemical, Japan), 3.5 parts SPconductive carbon black, 6.5 parts PVdF-HFP, and 11.0 parts PCplasticizer in about 90 parts acetone. Upon air-drying to remove theacetone coating vehicle, a flexible sheet of polymeric matrix positiveelectrode material was obtained. Electrode members of desired size,about 40 cm² ₁ were readily cut from this stock sheet. Two such sheetsof the material were pre-laminated to an interposed aluminum foil gridcurrent collector in the manner of Example I to yield a positiveelectrode member for the cell.

[0022] Negative electrode layer material was similarly prepared from ahomogeneous coating composition of 72.0 parts microbead mesophaseartificial graphite (MCMB 25-28-Osaka Gas Co., Japan), 2.5 parts SPconductive carbon black, 7.5 parts PVdF-HFP, and 18.0 parts PCplasticizer in about 90 parts acetone. Sized layers of the resultingmaterial were laminated to an interposed copper foil grid currentcollector to yield a negative electrode member for the cell.

[0023] The respective positive and negative electrode members wereincorporated into a unitary cell fabrication with an interveningseparator member sheet of 26 μm, 12.5 g/m² calendered regeneratedcellulose separator paper (CTF4826-NKK, Japan) by lamination in themanner of Example I at about 125° C. and 20 N/cm. The PC plasticizer ofthe laminated cell structure was removed by single extraction withdiethyl ether in the described manner followed by drying at about 100°C. under vacuum. The cell was then activated with 1 M electrolytesolution of LiPF₆ in a 1:1 mixture of ethylene carbonate anddimethylcarbonate (EC:DMC) and hermetically sealed in an envelope oftypical flexible packaging material. The activated cell of about 40 cm²was then subjected to C/2 and C/5 rate charge/discharge cycle testingbetween 2.8 V and 4.2 V and exhibited steady capacity of about 120-125mAh over the test period in excess of 100 cycles.

EXAMPLE VI

[0024] In a variant preparation procedure, a rechargeable battery cellfabricated by lamination in the manner of Example IV was heated undermild vacuum at about 70° C., as an alternative to extraction withselective solvent, in order to remove the processing plasticizer fromthe electrode matrix polymer and thereby reduce the cell'ssusceptibility to delamination under subsequent exposure to vagrantheat. The cell was then similarly activated with electrolyte solutionand packaged for operation prior to testing which provided substantiallyidentical results as the previous cell.

EXAMPLE VII

[0025] In another variant battery cell formulation, the positiveelectrode composition of Example V was revised by substitution of 10parts dibutyl phthalate (DBP) in place of the PC plasticizer componentof that earlier composition. The preparation of the cell electrodemembers was otherwise substantially similar. The resulting electrodemembers were assembled with a 50 μm, 15 g/m² kraft/manila fiber blendcapacitor separator paper (FLM 50/0.3-SPO Cascadec, Germany) andsubjected to the lamination conditions of that previous example. Theadhesion between the electrode and separator members was marginallyacceptable. A second set of electrode members was prepared from therespective electrode sheet materials. These electrode members wereassembled with a separator member of the same paper which had previouslybeen pre-treated with a 20% methyl alcohol solution of supplemental PCplasticizer according to the present invention as in Example IV and thelamination procedure was repeated. Interfacial adhesion of the cellmembers was satisfactory and, after extraction of plasticizers andactivation with electrolyte, the cell performed substantially as well asof that of Example V.

[0026] The substance of the present invention may be implemented toachieve thermal laminating adhesion between commercial paper fiberelectrochemical cell separator sheets and numerous combinations ofpolymer and copolymer matrix electrode compositions and compatiblesupplemental plasticizer materials. The wide range of suitable suchcomponent polymeric and plasticizer substances is apparent, for example,in the many useful, well-known compounds of this type which have beendescribed in the art, such as U.S. Pat. No. 5,540,741. The extent ofthese useful polymers and copolymer combinations of vinyl chloride,acrylonitrile, vinyl fluoride, vinyl acetate, styrene, vinylidenefluoride, and the like, and of the many applicable compatibleplasticizers for these polymeric materials, e.g., the above exemplarypropylene carbonate and mixtures of the same with homologous ethyleneand butylene carbonate, butyl adipate, cellosolve acetate, dimethylethers of diethylene or triethylene glycol, and the like, renderimpractical the recitation here of more than the foregoing smallexemplary number of such myriad combinations which are available for theadvantageous practice of the invention.

[0027] Suffice it to say that selection of suitable combinations of suchelectrode matrix polymers and compatible plasticizers are within theroutine capabilities of and subject to the preferences of the individualformulator or fabricator of desired electrochemical cell products, andthat such selection need only follow the described basic parameters ofthe invention which prescribe the introduction into theseparator-contiguous region of electrode polymer matrix composition asufficient amount of compatible supplemental plasticizer to render thecombination of these materials capable of physical adhesion to theseparator paper under desired heat and pressure conditions oflamination, as well as the preferred subsequent removal of at least aportion of the supplemental plasticizer, as by evaporation or selectiveextraction, in order to elevate the flow temperature of the compositionand thereby ensure against delamination under vagrant heatingconditions.

[0028] It is anticipated that other embodiments and variations of thepresent invention will become readily apparent to the skilled artisan inthe light of the foregoing description and examples, and it is intendedthat such embodiments and variations likewise be included within thescope of the invention as set out in the appended claims.

What is claimed is:
 1. An electrochemical cell system comprising:opposed planar electrode members composed of a polymeric matrixcomposition; and an interposed planar separator member; wherein saidseparator member is a matted fiber paper.
 2. The system of claim 1,further comprising a sufficient amount of a plasticizer compatible withsaid polymeric matrix composition to lower the inherent flow temperatureof said polymeric matrix composition and render said polymeric matrixcomposition capable of adhesive flow under preselected conditions ofheat and pressure.
 3. The system of claim 2, wherein said plasticizer isincorporated in said polymeric matrix composition.
 4. The system ofclaim 2, wherein said plasticizer is disposed in said separator member.5. The system of claim 1, wherein said matted fiber paper is selectedfrom the group consisting of single-layer and multilayer sheet productscomprising cellulose, regenerated cellulose, composite cellulose fibers,and mixtures thereof.
 6. The system of claim 1, wherein said polymericmatrix composition is selected from the group consisting of polymers andcopolymer combinations of vinylidene fluoride, hexafluoropropylene,chlorotrifluoroethylene, tetrafluoroethylene, vinyl fluoride, vinylchloride, vinyl acetate, styrene, and acrylonitrile.
 7. The system ofclaim 2, wherein said plasticizer is selected from the group consistingof propylene carbonate and mixtures thereof with ethylene carbonate andbutylene carbonate, butyl adipate, cellosolve acetate, dimethyl ethersof diethylene glycol and triethylene glycol, and mixtures thereof.
 8. Amethod of making an electrochemical cell system comprising the steps of:providing a pair of opposed planar electrode members composed of apolymeric matrix composition; providing a planar matted fiber paperseparator member interposed between said pair of electrode members; andlaminating said electrode and separator members together to form aunitary cell structure.
 9. The method of claim 8 wherein step ofproviding said electrode members comprises providing electrode memberscomposed of a polymeric matrix composition selected from the groupconsisting of polymers and copolymer combinations of vinylidenefluoride, hexafluoropropylene, chlorotrifluoroethylene,tetrafluoroethylene, vinyl fluoride, vinyl chloride, vinyl acetate,styrene, and acrylonitrile.
 10. The method of claim 8 wherein said stepof providing said separator member comprises providing a separatormember of said matted fiber paper selected from the group consisting ofsingle-and multi-layer sheet products comprising cellulose, regeneratedcellulose, composite cellulose fibers, and mixtures thereof.
 11. Themethod of claim 8, further comprising the steps of: providing asufficient amount of a plasticizer compatible with said polymeric matrixcomposition to lower the inherent flow temperature of said polymericmatrix composition and render said polymeric matrix composition capableof adhesive flow under preselected conditions of heat and pressure; andsaid step of laminating comprises applying said preselected conditionsof heat and pressure to said electrode and separator members.
 12. Themethod of claim 11 wherein said step of providing a plasticizercomprises providing a plasticizer selected from the group consisting ofpropylene carbonate and mixtures thereof with ethylene carbonate andbutylene carbonate, butyl adipate, cellosolve acetate, dimethyl ethersof diethylene glycol and triethylene glycol, and mixtures of theforegoing.
 13. The method of claim 11, wherein the step of providingsaid plasticizer comprises incorporating said plasticizer in saidpolymeric matrix composition.
 14. The method of claim 11, wherein thestep of providing said plasticizer comprises disposing said plasticizerin said separator member.
 15. The method of claim 11 wherein said stepof laminating comprises applying said preselected conditions of heat andpressure by means of a heated roller apparatus.
 16. The method of claim11 wherein said preselected condition of heat is in the range of about100° C.-140° C. and said preselected condition of pressure is in therange of 20-40 N/cm.
 17. The method of claim 11, wherein following saidstep of laminating, further comprising the step of removing saidplasticizer.
 18. The method of claim 17 wherein said step of removingcomprises removing said plasticizer by selective extraction with asolvent exhibiting negligible solvency toward said polymeric matrixcomposition.
 19. The method of claim 17 wherein said step of removingcomprises removing said plasticizer by evaporation.